feat: Implement go mod proxy strategy

[js-murph]

What

Implements a caching strategy for the Go module proxy as described in https://proxy.golang.org/

Why

To add support for caching go modules

Tests

Implemented unit tests. In addition manual testing was performed as follows...

1. Create a cachew.hcl:

memory {}

disk {
  root = "./state/cache"
}

gomod {
  proxy = "https://proxy.golang.org"
}

2. Run the proxy ./cachewd --config cachew.hcl

3. Manual verification...

$ time curl http://localhost:8080/github.com/alecthomas/kong/@latest
{"Version":"v1.13.0","Time":"2025-11-12T21:31:44Z","Origin":{"VCS":"git","URL":"https://github.com/alecthomas/kong","Hash":"d8de683664a2581e93717b7e4e8f4b55e4beeff4","Ref":"refs/tags/v1.13.0"}}
real	0m0.179s
user	0m0.006s
sys	0m0.011s

$ time curl http://localhost:8080/github.com/alecthomas/kong/@latest
{"Version":"v1.13.0","Time":"2025-11-12T21:31:44Z","Origin":{"VCS":"git","URL":"https://github.com/alecthomas/kong","Hash":"d8de683664a2581e93717b7e4e8f4b55e4beeff4","Ref":"refs/tags/v1.13.0"}}
real	0m0.033s
user	0m0.006s
sys	0m0.013s

[alecthomas]

Can this not be a sub-path? eg. /gomod/...?

I want to rethink how strategies get mounted, but in the interim I'd prefer to namespace them if possible. When we're mounting multiple strategies, it's hard to reason about how they'll interact.

[js-murph]

It can and should! I'll fix this.

[alecthomas]

I had another thought - I think we should enforce private repository blocking here, to prevent leaking to upstream public module proxies.

[alecthomas]

Which did also lead me to wonder whether we should be implementing a full proxy...but I think that's a question for another time.

[alecthomas]

Actually... https://github.com/goproxy/goproxy is explicitly designed to be embeddable, and has an extensible caching interface. Let's do some due diligence around whether this would be worthwhile to embed.

The advantage this could give us is the ability to cache private repositories.